Semiconductor wafers are typically batch processed in vacuum chemical vapor deposition (V-CVD) diffusion furnaces to form thin-film thereon of selected characteristics during various integrated circuit fabrication processes. The heretofore known V-CVD diffusion furnaces include a cylindrical diffusion tube having on one end an opening dimensioned to accept a batch of wafers to be processed, a tapering neck on its other end in communication with a controlled source of vacuum, and one or more gas-injection tubes slidably received in the tapering neck of the cylindrical diffusion tube. Reactant in gas phase injected into the diffusion furnace pyrolytically decomposes and deposits thin-film in solid phase not only on the semiconductor wafers, but also on the inside surface of the diffusion tube itself as well as on the surfaces of the one or more gas-injection tubes. The batch of coated wafers is then removed from the furnace, and the same or another process is run on another batch of wafers. With each such usage, the covering-thickness of the thin-film on the walls of the furnace and tubes increases, necessitating chemical etching to remove the build-up therefrom at regular intervals.
The heretofore known diffusion furnaces are disadvantageous due to the considerable labor and lost-revenue costs incurred each time it is necessary to remove the deposit build-up. For cleaning, the diffusion furnace is turned off to allow it to cool to room temperature. Thereafter, the diffusion tube and gas injection tubes are disconnected from the gas control subsystem, the vacuum subsystem, and the heating subsystem, and mechanically removed from the diffusion furnace. Prior to reconnection, air that has leaked into the disconnected gas control subsystem must be purged. The cleaned diffusion tube is then reconnected to the heating, vacuum, and gas subsystems. The one or more cleaned gas injection tubes are then reinserted and reconnected. Typically, the abovedescribed cleaning procedure takes fromm one to two days during which the furnace is inoperable with consequent loss of revenues.
The heretofore known diffusion furnaces are additionally disadvantageous in that the reconnection of the gas injection tubes is a comparatively complex procedure. Due to the difficulty of precisely realigning the gas injection tubes in the sloping walls of the tapering neck of the diffusion tube, injection tube reconnection is both a tedious and a time consuming procedure.
The heretofore known diffusion furnaces are further disadvantageous due to the fact that a diffusion tube having a tapering neck is itself an expensive component to fabricate, procure, repair, and replace. The tapering neck is comparatively fragile, and may break in use and during handling as a result of internal strain induced by both mechanical gas injection tube insertion stress and thermal stress during the operation of the furnace. Moreover, the replacement of a broken tube involves the same costly and time-consuming procedure used for furnace cleaning, and may possibly result in the contamination of a batch of wafers should breakage arise in use.